Processing seismic data

ABSTRACT

A method of matching the impulse response of a hydrophone and the impulse of a geophone accelerometer comprises performing a calculus operation upon the response of one of the hydrophone and the accelerometer. A filter is then derived from the output of the calculus operation and the response of the other of the hydrophone and the accelerometer. The filter may then be used to match seismic data acquired by the one of the hydrophone and the accelerometer to seismic data acquired by the other of the hydrophone and the accelerometer. The calculus operation may comprise differentiating the hydrophone response, or integrating the accelerometer impulse response.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national application of PCT/GB03/04935, filedNov. 13, 2003, which claims priority to a GB Application No. 0226675.7,filed Nov. 15, 2002. Each of the aforementioned related patentapplications is herein incorporated by reference.

The present invention relates to processing seismic data and, inparticular, to processing seismic data that includes two data sets, thedata sets relating to two different seismic parameters but to the samesurvey location.

In a seismic survey, seismic energy is emitted from a source and isdetected by a seismic receiver located at a distance from the source.Some of the seismic energy emitted by the source passes into the earth'sinterior and is reflected by geological structures within the earth.Information about the geological structure of the earth's interior canbe derived from the reflected seismic energy incident on the receiver.

A seismic receiver contains at least one sensor that detects seismicenergy. Various different types of seismic sensors are known. One commontype of seismic sensor is a hydrophone, which measures the pressure.Other known types of sensors, such as geophones, do not measure pressurebut instead measure a component of the particle motion (the term“particle motion” includes particle displacement, particle velocity,particle acceleration and, in principle, higher derivatives of particlevelocity). For example, a geophone measures the component of theparticle velocity along a particular direction (particle velocity is avector quantity, unlike pressure which is a scalar quantity). Athree-component geophone, or 3-C geophone, measures the components ofparticle velocity along three mutually perpendicular axes (which arenormally taken to be the x-, y- and z-axes).

Four component seismic receivers are known. A four component receiver,or 4-C receiver, is able to measure the pressure and three components ofthe particle velocity. A current 4-C receiver typically contains a 3-Cgeophone and a pressure sensor such as a hydrophone.

It is sometimes desired to combine measurements of different seismicparameters. For example, this may be done during the process of“de-ghosting” marine seismic data. In a marine seismic survey in whichthe receiver is located within a water column, in addition to thedesired paths of seismic energy that involve reflection at a structurewithin the earth, other seismic energy paths will occur as a result ofseismic energy being reflected or scattered from the surface of thewater column. These additional paths are known as “ghost reflections”.Ghost reflections are an undesirable source of contamination of seismicdata, since they obscure the interpretation of the desired up-goingreflections from the earth's interior.

In one known method, down-going ghost reflections are removed frommarine seismic data using the following filter:

$\begin{matrix}{\overset{\sim}{P} = {\frac{1}{2}\left\lbrack {P - {\frac{\rho\omega}{k_{z}}V_{z}}} \right\rbrack}} & (1)\end{matrix}$where {tilde over (P)} denotes the deghosted (up-going) pressure, P isthe measured pressure, V_(z) is the measured z-component of the particlevelocity (that is, the vertical component of the particle velocity),k_(z)=√{square root over (k²−κ²)} is the vertical wavenumber, κ²=k_(x)²+k_(y) ², and k_(x) and k_(y) are the horizontal wavenumbers.

Use of the filter of equation (1) requires pressure data and particlevelocity data to be combined, in this example by subtraction. Thereexist other cases in which pressure data and particle velocity data arecombined, for example by addition or subtraction. As an example, methodsfor decomposing an acquired wavefield, for example into up-going anddown-going constituents, are known that involve combination of pressuredata and particle velocity data.

When seismic data relating to two different seismic parameters are to becombined, for example by addition, subtraction etc, it is usuallynecessary to match the two sets of data to one another before they canbe combined. This is because a seismic data acquired using one type ofsensor (such as a geophone) may not be directly combinable with seismicdata acquired using a different type of sensor (such as a hydrophone).For example, the amplitude response of a geophone, as a function offrequency of the received seismic energy, may be different from theamplitude response of a hydrophone.

The phase response of a geophone, as a function of frequency of theseismic energy, may also be different from the phase response of ahydrophone. As a result, it is necessary to carry outfrequency-dependent phase matching and frequency-dependent amplitudematching before geophone data (which is commonly, but not necessarilyvelocity data) can be combined with hydrophone (pressure) data. Thismatching may be carried out using, for example, a filter that attemptsto match the impulse response of the geophone with the impulse responseof the hydrophone. A typical matching filter is shown in FIG. 1.

It is expected that accelerometer sensors will increasingly often beused to acquire particle motion data, instead of conventional geophones.Accelerometer measurements are closely proportional to particleacceleration, rather than particle velocity, over most of the frequencyband of interest in seismic surveying. Since accelerometer andhydrophone data are different types of measurements, a hydrophone and anaccelerometer will experience an impulse very differently. According tothe equation of motion in an acoustic medium, a time derivative ofparticle velocity is proportional to the gradient (first-order spatialderivatives) of pressure. Hydrophone data therefore have a magnituderesponse that is more proportional to particle velocity rather than toparticle acceleration.

In order to combine accelerometer data and hydrophone (pressure) data itis again necessary to use a filter to match the phase and/or amplituderesponses of the accelerometer and the pressure sensor. The filter shownin FIG. 1 is intended to be used in matching accelerometer data withpressure (hydrophone) data. However, direct matching of accelerometerdata and pressure data, by a matching filter that matches the impulseresponse of an accelerometer and the impulse response of a hydrophone,suffers from edge effects. These edge effects arise because theamplitude response and phase response of an accelerometer are verydifferent to the amplitude response and phase response of a hydrophone.

This problem is illustrated shown in FIGS. 2( a) to 2(c), whichillustrate the matching of an accelerometer impulse response to ahydrophone impulse response using the filter of FIG. 1. In the matchingprocess the accelerometer impulse response is filtered using the filterof FIG. 1, and the intention is that the filtered accelerometer impulseresponse should be matched to the hydrophone impulse response.

FIG. 2( a) shows the amplitude response of a typical geophoneaccelerometer, and FIG. 2( b) shows the amplitude response of a typicalhydrophone. The matched accelerometer response, obtained by applying thematching filter of FIG. 1 to the geophone response of FIG. 2( a), isshown in FIG. 2( c) and it will be seen that this is not identical tothe hydrophone response of FIG. 2( b) FIG. 3 illustrates the differencebetween the hydrophone's impulse response of FIG. 2( b) and the filteredaccelerometer response of FIG. 2( c). Edge effects resulting from usinga direct matching filter may be seen at low and high sample numbers, andthese indicate that the matching filter has not correctly matched theaccelerometer impulse response to the hydrophone impulse response.Ideally, the filtered accelerometer impulse response should match thehydrophone impulse response.

WO 00/55648 discloses a hydrophone assembly having a hydrophone and anin-built differentiator. The differentiator modifies the frequencyresponse of the hydrophone, so that the frequency response of thehydrophone assembly more closely matches the frequency response of anaccelerometer.

A first aspect of the present invention provides a method of matchingthe response of a hydrophone and the response of an accelerometer, themethod comprising the steps of: performing a calculus operation upon theresponse of at least one of the hydrophone and the accelerometer; andderiving a filter from the output of the calculus operation and theresponse of the other of the hydrophone and the accelerometer.

The present invention involves the two steps of, firstly, performing acalculus operation upon the response of one of the hydrophone and theaccelerometer and, secondly, deriving a filter that matches this withthe response of the other of the hydrophone and the accelerometer. Theinvention provides better matching of the frequency and phase responseof a hydrophone to the frequency and phase response of an accelerometerthan does WO 00/55648, which uses only the step of differentiating thehydrophone output.

In practice, the output of an accelerometer may not be exactlyproportional to the particle acceleration over the entire frequencyrange used in a seismic survey and, similarly, the output of ahydrophone may not be exactly proportional to the particle velocity overthe entire frequency range used in a seismic survey. WO 00/55648 isunable to allow for this, since the differentiator of WO 00/55648 ishard-wired into the hydrophone assembly and operates on data of allfrequencies. The present invention is able to allow for this, incontrast, since it incorporates use of the filter of the invention anddoes not simply apply the calculus operation to the response of the oneof the hydrophone and accelerometer.

Furthermore, the frequency response of one type of accelerometer mayvary from the frequency response of another type of accelerometer.Indeed, two accelerometers of the same type may have frequency responsesthat vary from one another. The present invention is able to takeaccount of such variations, through the filter. In contrast, WO 00/55648cannot take account of such variations, since the differentiator ishard-wired into the hydrophone assembly so that the frequency responseof the hydrophone assembly is fixed.

According to the invention a calculus operation is applied to oneresponse before determination of the matching filter. In one preferredembodiment of the invention, for example, the accelerometer response isintegrated before determination of the matching filter. The originalaccelerometer data is, as noted above, closely proportional to particleacceleration over most of the frequency band of interest in seismicsurveying, so the integrated accelerometer response should be closelyproportional to particle velocity—and so may be more accurately matchedwith a hydrophone response which is also proportional to particlevelocity.

In an alternative preferred embodiment of the invention, the hydrophoneresponse is differentiated before matching with the accelerometerresponse. The original hydrophone data is proportional to particlevelocity, so the derivative of the hydrophone response should beproportional to particle acceleration. The derivative of the hydrophoneresponse may therefore be accurately matched with the accelerometerresponse.

The present invention thus allows the responses of a hydrophone and ageophone, which are two inherently different types of seismic sensor, tobe matched to one another. This allows a set of seismic data obtained bya hydrophone and a set of seismic data obtained a geophone to becombined with minimum introduction of edge effects.

A second aspect of the invention provides a method of processing seismicdata comprising the steps of: obtaining a filter for matching theresponse of an accelerometer and the response of a hydrophone accordingto a method of the first aspect; obtaining first seismic data using theone of the hydrophone and the accelerometer and obtaining second seismicdata using the other of the hydrophone and the accelerometer; and usingthe matching filter to match the first seismic data to the secondseismic data. In principle, the filter may be obtained before or afterthe seismic data are obtained.

The matching process may comprise applying the calculus operation to thefirst seismic data and subsequently applying the matching filter to thefirst seismic data. Alternatively, the filter may be used to derive oneor more other filters which are then used to match the first seismicdata to the second seismic data. After matching, the first seismic datamay be combined with the second seismic data.

A third aspect of the invention provides an apparatus for matching theresponse of a hydrophone and the response of an accelerometer, theapparatus comprising: means for performing a calculus operation upon theresponse of at least one of the hydrophone and the accelerometer; andmeans for deriving a filter from the output of the calculus operationand the response of the other of the hydrophone and the accelerometer.

A fourth aspect of the invention provides an apparatus for processingseismic data and comprising: means for receiving first seismic dataacquired using one of a hydrophone and an accelerometer and secondseismic data acquired using the other of the hydrophone and theaccelerometer; and means for matching the first seismic data and thesecond seismic data using a matching filter obtained by a method of thefirst aspect of the invention.

Further aspects and preferred features of the invention are set out inthe remaining claims.

Preferred embodiments of the present invention will now be described byway of illustrative example with reference to the accompanying figures,in which:

FIG. 1 shows a filter intended to match an accelerometer impulseresponse to a hydrophone impulse response;

FIG. 2( a) shows a typical geophone accelerometer impulse response;

FIG. 2( b) shows a typical hydrophone impulse response;

FIG. 2( c) shows the result of applying the matching filter of FIG. 1 tothe accelerometer impulse response of FIG. 2( a);

FIG. 3 shows the difference between the filtered accelerometer impulseresponse of

FIG. 2( c) and the hydrophone impulse response of FIG. 2( b);

FIG. 4 shows a filter intended to match an integrated accelerometerimpulse response to a hydrophone impulse response;

FIG. 5 shows an integrated accelerometer impulse response obtained byintegrating the accelerometer impulse response of FIG. 2( a);

FIG. 6 shows the difference between the filtered accelerometer impulseresponse of FIG. 5 and the hydrophone impulse response of FIG. 2( b);

FIG. 7( a) is a flow diagram of one method for obtaining a matchingfilter according to the present invention;

FIG. 7( b) is a flow diagram of another method for obtaining a matchingfilter according to the present invention;

FIG. 8 is a flow diagram of one method for processing seismic dataaccording to a method of the present invention; and

FIG. 9 is a block circuit diagram of an apparatus according to thepresent invention.

According to the invention a matching filter is derived for use inmatching a first set of seismic data acquired by one of a hydrophone andan accelerometer to a second set of seismic data acquired by the otherof the hydrophone and the accelerometer. The matching filter is derivedfrom the responses of the hydrophone and accelerometer. One methodaccording to the invention for deriving the matching filter is shown inFIG. 7( a).

The input required to determine the matching filter is the impulseresponse of the hydrophone and the impulse response of theaccelerometer. These may be derived from, for example, performance datasupplied by the manufacturer. In general, a manufacturer will supplyperformance data with a hydrophone or accelerometer, and these data willinclude a theoretical impulse response. The data may include a generaltheoretical impulse response for the particular type of hydrophone oraccelerometer. Alternatively, the data supplied by the manufacturer maygive a theoretical impulse response for a device as a function of deviceparameters such as, for example, the resonant frequency. In this casethe user may measure the relevant parameter(s) for a particular sensorand use these to determine the impulse response for the particularsensor.

In other cases, the manufacturer may determine the actual impulseresponse for each individual hydrophone or accelerometer, and supplythese data to the user with the accelerometer/hydrophone. As a furtheralternative, the user may determine the impulse response for aparticular hydrophone and/or accelerometer.

Thus, the input to the method of FIG. 7( a) is the impulse response ofthe hydrophone Res_(H), at step A, and the impulse response of theaccelerometer Res_(A), at step C. Each impulse response may be an actualresponse or the best available estimate for the response.

According to the invention, a calculus operation is applied to theresponse of one of the hydrophone and the accelerometer. In theembodiment of FIG. 7( a), a differentiating operator is applied to theresponse of the hydrophone, at step B. The differentiation may be in thetime domain or in the frequency domain. The output from step B is thederivative of the hydrophone response Res_(H), and this will be denotedby diff(Res_(H)).

At step D a matching filter is determined from the result of thecalculus operation B and from the impulse response of the otherdevice—namely the device whose impulse response was not involved in thecalculus operation at step B, which, in this embodiment, is theaccelerometer.

In a preferred embodiment, the matching filter F₁ is determined bydividing the differentiated impulse response of the hydrophone by theimpulse response of the accelerometer. That is, the filter F₁ isdetermined according to:F ₁=diff(Res_(H))/Res_(A)  (2)

The matching filter F₁ may then be used to process seismic data in theacceleration domain, as will be described in more detail below.

FIG. 7( b) shows another embodiment of the method of the invention forobtaining a matching filter. In the embodiment, a calculus operation isagain applied to the response of one of the hydrophone and theaccelerometer, but in the embodiment of FIG. 7( b), an integrationoperator is applied to the response of the accelerometer, at step E. Theintegration may be performed in the time domain or in the frequencydomain. The output from step E is the integral of the accelerometerresponse Res_(A), and this will be denoted as Int (Res_(A)).

At step G a matching filter F₂ is determined from the result of thecalculus operation D and from the impulse response of the otherdevice—which, in this embodiment, is the hydrophone.

In a preferred embodiment, the matching filter F₂ is determined at stepG by dividing the integrated impulse response of the accelerometer bythe impulse response of the hydrophone. That is, the filter F₂ isdetermined according to:F ₂=Int(Res_(A))/Res_(H)  (3)

The matching filter F₂ may then be used to process seismic data in thevelocity domain, as will be described in more detail below.

A matching filter of the invention may be used to match a set of seismicdata obtained using an accelerometer to another set of seismic dataobtained at the same survey location using a hydrophone. A calculusoperation is applied to one of the sets of seismic data. Morespecifically, the same calculus operation as used to determine thematching filter is applied to seismic data acquired by the sensor towhose response the calculus operation was applied during thedetermination of a matching filter. The appropriate matching filter isthen applied to match the amplitude and/or phase of that data set to theamplitude and/or phase of the other data set.

In one preferred embodiment of the matching process of the invention,the accelerometer data are integrated and the matching filter F₂ isapplied to the integrated accelerometer data to match them to thehydrophone data. The original accelerometer data is, as noted above,closely proportional to particle acceleration over most of the frequencyband of interest in seismic surveying, so that the integratedaccelerometer data should be closely proportional to particlevelocity—and so may be more accurately matched with the hydrophone datawhich is also proportional to particle velocity.

In another preferred embodiment of the matching process of theinvention, the hydrophone data are differentiated and the matchingfilter F₁ is applied to the differentiated hydrophone data to match themto the accelerometer data.

FIGS. 4 to 6 illustrate results obtained by the invention. FIG. 5 showsthe results of integrating the geophone accelerometer response of FIG.2( a) with respect to time. In this embodiment the integration wascarried out using a Laplace technique but, in principle, the integrationmay be carried out using any suitable numerical integration technique.FIG. 5 thus represent the results of step E of the method of FIG. 7( b).

FIG. 4 shows a matching filter for matching the integrated accelerometerresponse of FIG. 5 to the hydrophone response of FIG. 2( b). FIG. 4represents the filter F₂ obtained by step G of the method of FIG. 7( b).In this case the filter was obtained by dividing the integratedaccelerometer response of FIG. 5 by the hydrophone response of FIG. 2(b), but it may alternatively be obtained using any suitable numericaltechnique.

FIG. 6 illustrates the difference between the matched integratedaccelerometer impulse response, obtained by filtering the integratedaccelerometer response of FIG. 5 using the filter of FIG. 4, and thehydrophone impulse response of FIG. 2( b). It will be seen, by comparingFIGS. 3 and 6, that the present invention provides a much smallerdifference between the two impulse responses that does the conventionalmatching process. In FIG. 6 the method of the present invention providesa difference between the impulse responses that, at the edges, reaches amaximum of approximately −100 dB. In contrast, as shown in FIG. 3, theconventional method produces a difference between the impulse responsesthat, at the edges, reaches a maximum of close to 0 dB. (It will benoted that the vertical scale of FIG. 6 covers smaller amplitudedifferences that does the vertical scale of FIG. 3.)

(In principle, the result of applying the matching filter of FIG. 4 tothe integrated accelerometer response of FIG. 5 would be identical tothe hydrophone response of FIG. 2( b), since the matching filter wasderived from the integrated accelerometer response and the hydrophoneresponse. Limitations such as rounding errors in the integration stepand numerical errors in the determination of the filter mean howeverthat, in practice, the integrated accelerometer response is notidentical to the hydrophone response, as indicated in FIG. 6.)

Once the matching filter F₂ has been obtained, two sets of seismic datamay then be combined. For example, if the data are acquired in a marineseismic survey and accelerometer data represent the vertical componentof particle motion, the accelerometer data and hydrophone data may bematched by integrating the accelerometer data and applying the matchingfilter as described above. The matched, integrated accelerometer dataand the hydrophone data may then be combined according to equation (1)to remove down-going ghost reflections.

Once the two data sets have been combined, the combined data may besubjected to further processing steps. For example, in the example givenin the preceding paragraph the de-ghosted data obtained by combining theaccelerometer data and hydrophone data may be subjected to anyconventional processing steps to obtain information about the geologicalstructure of the earth's interior.

In a second embodiment of the matching process of the invention one setof seismic data again contains particle motion data obtained using anaccelerometer, and the other set of seismic data contains pressure dataobtained using a hydrophone. In the second embodiment of the inventionthe hydrophone data are differentiated. The differentiation may again becarried out in the time domain or in the frequency domain, and it may becarried out using any suitable numerical technique. The matching filterF₁ is then applied to the differentiated set of pressure data, so as tomatch the differentiated set of pressure data to the set ofaccelerometer data.

The matched data sets may then be combined, as described above, and thecombined data may be subjected to further data processing steps.

FIG. 8 is a flow diagram of processing seismic data according to oneembodiment of the present invention.

Initially a set of seismic data is acquired at step 1, and another setof seismic data is acquired at step 2. These sets of seismic data areacquired at the same survey location, but the one set of data isacquired using a hydrophone and the other set of data is acquired usingan accelerometer. Although steps 1 and 2 are shown as occurringconsecutively in FIG. 7 this is only for ease of description. Inpractice steps 1 and 2 may be carried out simultaneously, for exampleusing a seismic receiver that contains two or more different types ofseismic sensor.

Alternatively, the invention may be applied to pre-existing seismicdata. In this case steps 1 and 2 are replaced by the step, step 3, ofretrieving two sets of seismic data from storage. The two retrieved setsof seismic data were again acquired at the same survey location, butusing a hydrophone and an accelerometer respectively, and representdifferent seismic quantities.

As a further alternative, the method may be applied to sets of syntheticseismic data. Steps 1 and 2, and step 3 may therefore be replaced by thestep, step 4, of synthesizing the two sets of seismic data or retrievingpreexisting sets of synthetic seismic data from storage.

Next, at step 5, a calculus operation is applied to one of the datasets. For a particular data set, the calculus operation to be applied atstep 5 is the same as the calculus operation applied to the response ofthe sensor used to acquire that data in the determination of a matchingfilter according to FIG. 7( a) or 7(b). That is, step 5 may consist ofapplying a differential operator to the set of hydrophone data or it mayconsist of applying an integration operator to the set of accelerometerdata. Step 5 may be carried out using any suitable numerical integrationor differentiation technique. The calculus operation may be performed inthe time domain or in the frequency domain. (It is preferable, althoughnot essential, that the calculus operation on the data is carried out inthe same domain as the calculus operation used to obtain the filter,since unwanted artefacts may otherwise occur.)

At step 6 the appropriate matching filter of the invention is applied tothe data set to which the calculus operation was applied at step 5. Ifstep 5 consists of applying a differential operator to the set ofhydrophone data, then step 6 consists of applying the filter F₁,obtained according to the method of FIG. 7( a), to the differentiatedhydrophone data. Conversely, if step 5 consists of applying anintegration operator to the set of accelerometer data, then step 6consists of applying the filter F₂, obtained according to the method ofFIG. 7( b), to the integrated accelerometer data.

The two sets of data may then be combined at step 7, and the combineddata may be subjected to further processing steps (denoted schematicallyas step 8).

In the embodiments described above a calculus operation is performed onthe response of one of the sensors, and the result of this operation isused to derive the matching filter. This filter is then applied to dataacquired by that sensor (i.e., the sensor to whose response the calculusoperation was applied.) However, the invention is not limited to this.

For example, if a filter is derived from the integrated accelerometerresponse, as in FIG. 7( b), the matching process does not necessarilyinvolve applying that filter to integrated accelerometer data. Thematching could be effected by, for example, using the filter obtainedfrom the integrated accelerometer response to derive a second filterwhich can be applied to the hydrophone data so as to match it to theintegrated accelerometer data. The second filter would be, in a generalsense, an inverse of the filter derived from the integratedaccelerometer response. A similar procedure may also be used where thefilter is initially derived from the differentiated hydrophone response.

In principle, it would even be possible to derive two filters from thefilter derived from the integrated accelerometer response or from thedifferentiated hydrophone response. In this case the matching processwould comprise applying one filter to the integrated accelerometer dataand applying the other filter to the hydrophone data. It should benoted, however, that this is likely to be more computationally intensivethan a matching process that uses only a single filter.

Furthermore, the embodiments described above involve only a singlecalculus operation. This is applied to the response of either theaccelerometer or the hydrophone, and the effect of applying the calculusoperation is to put both responses into a common domain and therebyenable efficient matching. Once this has been done however, it would bepossible to apply a further calculus operation to both responses, totransfer them both to a desired, different domain. In a modification ofthe embodiment of FIG. 7( a), for example, a further calculus operation(not shown) could be applied both to the differentiated hydrophoneresponse and to the accelerometer response to transfer them both toanother domain. In this modified embodiment, the further calculusoperator applied to the hydrophone response may be combined with thedifferentiating operator B so that the hydrophone response is operatedon by only a single calculus operator or, alternatively, the furthercalculus operation may be applied as a separate operation after, orpossibly before, the differentiating operator B is applied. Theembodiment of FIG. 7( b) may be modified in an analogous way.

FIG. 9 is a schematic block diagram of an apparatus 11 that is able toperform a method according to the present invention.

The apparatus 11 comprises a programmable data processor 12 with aprogram memory 13, for instance in the form of a read only memory (ROM),storing a program for controlling the data processor 12 to processseismic data by a method of the invention. The apparatus furthercomprises non-volatile read/write memory 14 for storing, for example,any data which must be retained in the absence of a power supply. A“working” or “scratch pad” memory for the data processor is provided bya random access memory RAM 15. An input device 16 is provided, forinstance for receiving user commands and data. One or more outputdevices 17 are provided, for instance, for displaying informationrelating to the progress and result of the processing. The outputdevice(s) may be, for example, a printer, a visual display unit, or anoutput memory.

Sets of seismic data for processing may be supplied via the input device16 or may optionally be provided by a machine-readable data store 18.

The results of the processing may be output via the output device 17 ormay be stored.

The program for operating the system and for performing the methoddescribed hereinbefore is stored in the program memory 13, which may beembodied as a semiconductor memory, for instance of the well known ROMtype. However, the program may well be stored in any other suitablestorage medium, such as a magnetic data carrier 13 a (such as a “floppydisk”) or a CD-ROM 13 b.

1. A method of matching the response of a hydrophone and the response ofan accelerometer, the method comprising the steps of: performing acalculus operation upon the response of at least one of the hydrophoneand the accelerometer; and deriving a filter from the output of thecalculus operation and the response of the other of the hydrophone andthe accelerometer by dividing a result of the calculus operation by theresponse of the other of the hydrophone and the accelerometer.
 2. Amethod as claimed in claim 1 wherein the step of performing the calculusoperation comprises the step of integrating the response of theaccelerometer with respect to time.
 3. A method as claimed in claim 1wherein the step of performing the calculus operation comprises the stepof differentiating the response of the hydrophone with respect to time.4. A method as claimed in claim 1 further comprising: obtaining firstseismic data using the one of the hydrophone and the accelerometer andobtaining second seismic data using the other of the hydrophone and theaccelerometer; and using the filter to match the first seismic data andthe second seismic data.
 5. A method as claimed in claim 1 furthercomprising: synthesizing first seismic data for the one of thehydrophone and the accelerometer and synthesizing second seismic datafor the other of the hydrophone and the accelerometer; and using thefilter to match the first seismic data to the second seismic data.
 6. Amethod as claimed in claim 4 or 5 and further comprising: applying thecalculus operation to the first seismic data; and wherein the step ofusing the filter to match the first seismic data to the second seismicdata comprises applying the filter to the first seismic data after thecalculus operation has been applied to the first seismic data.
 7. Amethod as claimed in claim 4 and further comprising: combining thematched first seismic data and the second seismic data.
 8. A method asclaimed in claim 7 and comprising the further step of applying one ormore data processing steps to the combined seismic data.
 9. An apparatusfor matching the response of a hydrophone and the response of anaccelerometer, the apparatus comprising: means for performing a calculusoperation upon the response of at least one of the hydrophone and theaccelerometer; and means for deriving a filter from the output of thecalculus operation and the response of the other of the hydrophone andthe accelerometer by dividing a result of the calculus operation by theresponse of the other of the hydrophone and the accelerometer.
 10. Anapparatus as claimed in claim 9 and further comprising: means forreceiving first seismic data acquired using the one of a hydrophone andan accelerometer and second seismic data acquired using the other of thehydrophone and the accelerometer; and means for matching the firstseismic data and the second seismic data using the filter.
 11. Anapparatus as claimed in claim 10 and further comprising: means forapplying the calculus operation to the first seismic data; and means forsubsequently applying the filter to the first seismic data.
 12. Anapparatus as claimed in claim 10 and further comprising means forcombining the first seismic data and the second seismic data.
 13. Anapparatus as claimed in claim 10 comprising a programmable dataprocessor.
 14. A storage medium containing a program configured to:perform a calculus operation upon the response of at least one of ahydrophone and an accelerometer; and derive a filter from the output ofthe calculus operation and the response of the other of the hydrophoneand the accelerometer by dividing a result of the calculus operation bythe response of the other of the hydrophone and the accelerometer. 15.The storage medium of claim 14, wherein the program configured toperform the calculus operation is further configured to integrate theresponse of the accelerometer with respect to time.
 16. The storagemedium of claim 14 wherein the program configured to perform thecalculus operation is further configured to differentiate the responseof the hydrophone with respect to time.
 17. The storage medium of claim14, wherein the program is further configured to: obtain first seismicdata using the one of the hydrophone and the accelerometer: obtainsecond seismic data using the other of the hydrophone and theaccelerometer; and use the filter to match the first seismic data andthe second seismic data.
 18. The storage medium of claim 14, wherein theprogram is further configured to: synthesize first seismic data for theone of the hydrophone and the accelerometer; synthesize second seismicdata for the other of the hydrophone and the accelerometer; and use thefilter to match the first seismic data to the second seismic data. 19.The storage medium of claim 17, wherein the program is furtherconfigured to: apply the calculus operation to the first seismic data;and wherein the program configured to use the filter to match the firstseismic data to the second seismic data is further configured to applythe filter to the first seismic data after the calculus operation hasbeen applied to the first seismic data.